One of the common difficulties in an optical information recording disk, e.g., laser disk, relates to the occurrence of focussing errors. An astigmatic method has been introduced to solve the problem.
In FIG. 1, there is illustrated a prior art optical pickup system 10 utilizing the astigmatic method, as disclosed in U.S. Pat. No. 4,023,033, entitled "Optical Focussing Device" and is incorporated herein by reference. The optical pickup system 10 comprises a light source 12, a beam splitter 16, an objective lens 18, an optical information recording disk 20 (hereinafter, referred to as an optical disk), a cylindrical lens 26 and an optical detector 28. In the system, a light beam 14 emitted from the light source 12, e.g., a laser diode, enters the beam splitter 16 and is partially reflected by a reflection surface 22 incorporated therein. The light beam reflected from the reflection surface 22 is radiated through the objective lens 18 onto a recording surface 21 of the optical disk 20 as a focussed light beam. The focussed light beam reflected from the optical disk 20 is converged by the objective lens 18 and transmitted through the beam splitter 16. The focussed light beam transmitted through the beam splitter 16 is made astigmatic by its passage through the cylindrical lens 26 and thereafter is made to impinge onto the optical detector 28 including a light-reception surface 29 having four square photoelectric cells(not shown) arranged to form a square. Each of the photoelectric cells generates an output in the form of a light intensity measurement. Two outputs from two opposite corners of the square light-reception surface are sent to a first adder and two outputs from the remaining two opposite corners are sent to a second adder, respectively. Results from the first and second adders are then sent to a differential amplifier (not shown) which will in turn generate an associated focussing error by comparing the outputs from the first and second adders, the focussing error simply being a difference of the two outputs from the pair of adders.
Being astigmatic, the shape of the luminous flux imaged on the light-reception surface 29 of the optical detector 28 changes depending on the positional relationship between the recording surface 21 of the optical disk 20 and a convergence point 19 of the light beam. In order to detect this change in the shape of the luminous flux, the cylindrical lens 26 is arranged exactly between the convergence point 19 and the optical detector 28 in such a way that the light-reception surface 29 is disposed at the position where the shape of the luminous flux becomes circular when the light beam is focussed (zero focussing error) and this is known a "just focussed" position. If the optical disk 20 is displaced in a vertical direction from the just focussed position to the optical detector 28, the focussing error signal becomes non-zero with the sign indicating the direction of displacement, thereby detecting the focussing error. This conventional astigmatism method requires a cylindrical lens to focus a beam spot in an astigmatic manner. Since, however, the conventional cylindrical lens employed therein is not planar, it is rather difficult to align it accurately with the convergence point 19 and the optical detector 28 such that the light-reception surface 29 is disposed at the position where the shape of the luminous flux becomes circular.